Ground outside hard, but provided with boots (4)
I believe the answer is:
shod
'provided with boots' is the definition.
(shod can mean given or wearing shoes)
'ground outside hard' is the wordplay.
'ground' becomes 'sod' (sod is a kind of ground).
'outside' is an insertion indicator.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'sod' enclosing 'h' is 'SHOD'.
'but' acts as a link.
(Other definitions for shod that I've seen before include "Exclamation used to frighten animals away" , "with hooves protected?" , "Not with bare feet" , "Fitted with footwear" , "Fitted with horseshoes" .)